A detailed understanding of the neurophysiological basis of hearing is fundamental to the understanding of human hearing impairment and the guidance of further development of the most successful prosthetic intervention to date, the cochlear implant. Yet we still lack a complete description of how sound information is processed at even the first central nervous system relay, the cochlear nucleus. Different aspects of sound are extracted from the auditory nerve spike trains and encoded via parallel neural pathways. While the coding of timing cues have been studied extensively, the processing of intensity cues remains unclear, especially relating to non-localization tasks such as sound recognition. We recently determined that the timing and intensity circuits in the cochlear nucleus are distinguished by the expression of different forms of short-term synaptic plasticity. In vitro studies have demonstrated that the intensity pathways exhibit a mixture of short-term facilitation and depression that allows the transmission of rate-encoded intensity information. In contrast, the short-term depression found in timing circuits creates a synaptic gain control that contributes to intensity-invariant coding of timing cues. We expand our investigation of intensity coding to spike trains in response to dynamic, amplitude modulated sounds, an important component of sound communication signals. The goal of this proposal is to identify the synaptic and cellular mechanisms that contribute to the encoding of sound intensity and establish their importance in the intact brain. Aim 1 uses the avian (chick) cochlear nucleus slice preparation to investigate two synaptic enhancement mechanisms: short-term synaptic facilitation and the contribution of NMDA-receptor currents to synaptic integration. We use dynamic clamp to determine the input-output function of cochlear nucleus neurons. Aim 2 investigates how dynamic stimuli like amplitude-modulated sounds are processed at auditory nerve synapses in the cochlear nucleus by measuring physiological synaptic responses to rate-modulated spike train inputs, using electrical stimulation and dynamic clamp. In Aim 3, we will extend our in vitro short-term plasticity results to the intact cochlear nucleus with in vivo, intracellular recordings in the avian brainstem. Given the recent advances in the restoration of hearing using prosthetic devices that stimulate the auditory nerve, it is critical to understand how nerve activity is interpreted by the central nervous system. This proposal will provide new information on the transformation of auditory information which will help improve assisted-hearing devices and lead to a better understanding of normal hearing.